1. Field of the Invention
The present invention relates to plasma spraying process and more particularly to plasma arc apparatus for applying mostly metal coatings to works of various configuration, especially to elongated works.
Most advantageously the invention may be used to apply strengthening, decorative and other coatings in tool producing industry, medicine, jewelry and other fields.
2. Description of the Prior Art An important problem encountered by those skilled in the art when developing plasma arc apparatus for applying coatings consists in improving the quality of a surface of a part subjected to spraying process ensuring required physical and mechanical properties of the latter. The main factor which prevents the solution of this problem is the presence of electrically neutral macroparticles: drops and solid fragments of the evaporating material of the cathode in the plasma stream produced by this material. The above macroparticles deteriorate the surface of the part being treated. To prevent this, magnetic separation is used in order to separate the plasma stream into neutral and charged particles so that a part could be treated by charged particles only.
Known in the art is a plasma arc apparatus wherein the separation of the plasma stream occurs when the plasma stream moves in a curvilinear plasma guide provided with a magnetic system (see I. I. Aksenov, V. A. Belous et al. Ustroystvo dly ochistki plasmy vakuumnoi dugi ot makrochastits. "Pribory i tekhnika eksperimenta", M., 1978, No. 5, s. 236-237). When using this apparatus, the neutral particles are deposited on the walls of the plasma guide, whereas the charged particles (positive ions) are diverted by the magnetic system and are directed through the plasma guide to the work being treated.
A substantial disadvantage of this apparatus consists in considerable losses of charged particles which are deposited on the walls of the plasma guide because of energetic losses and because of an extended length of the latter.
There is also known in the art another plasma arc apparatus for applying coatings, comprising a working anode chamber of a non-magnetic metal, a consumable cathode disposed in this chamber whose evaporation surface faces the anode, a starting electrode contacting the cathode, a direct current source for feeding an arc, a pulsed current source for starting the arc and a magnetic system mounted on the anode (see V. A. Osipov, V. G. Padalka et al. Ustanovka dly nanesenia pokryty osazhdeniem ionov, izvlekayemykh iz plazmy vakuumnoi dugi. "Pribory i tekhnika eksperimenta", M., 1978, No. 6, s. 173-175).
In this apparatus the direct current source for feeding the arc is connected with the cathode and anode, and the pulsed current source for starting the arc is connected with the cathode and the starting electrode. Said magnetic system is intended for diverting charged particles of plasma from the surface of the anode in the direction of the work being treated. Te butt-end surface of the cathode is the evaporation surface thereof. The apparatus is provided with an additional magnetic system intended to maintain the cathode spot on the butt-end surface of the cathode (this system is described in detail in USSR Inventor's Certificate No. 307,666, published in 1979). The working anode chamber of the apparatus is conically shaped and is provided in the butt-end with a cover. On the inner surface of the cover there are installed works to be treated. There is also provided a cup disposed adjacently the central portion of the cover. In this cup the above cathode is disposed made in the form of a relatively short cylinder with a rod. The butt-end of the cathode is disposed practically in one plane with the inner surface of the cover.
In operation of the described apparatus the plasma stream comprising vapours of the material of the cathode is directed to the anode. The working conditions of the apparatus are selected such that the positive ions contained in the stream are diverted by the electric field produced in the plasma under the influence of the magnetic field in the region of the conical surface of the anode and are directed towards the cover. These positive ions are deposited on the works being treated while the neutral macroparticles are deposited on the inner surface of the anode.
In the course of operation of this apparatus there occur certain difficulties which prevent an extensive use thereof. Thus, the plasma spraying process carried out by this apparatus can be used for treating but one surface of a work. To treat another surface of the same work, it is necessary to switch off the apparatus and to turn the work over either manually or with the aid of a special mechanism. For this same reason it is rather difficult to treat works in the form of bodies of revolution.
In addition, because of small area of the working surface of the cover it is impossible to treat elongated works, such as rods, strips, cores and bands.
It is also to be noted that, since the evaporation surface in the above-described apparatus is the butt-end thereof whose area is usually not more than 200 cm.sup.2, the efficiency of the apparatus is substantially reduced, which makes it necessary to increase the consumption of electric energy. This latter necessity detrimentally affects the process of evaporation of the cathode material and, consequently, leads to rapid consumption thereof, frequent replacement of the cathode and to deterioration of the quality of the surface subjected to plasma spraying process.